The known parachuting disciplines related to free fall can be divided into those that are performed with a constant rate of fall (about 50-55 m/s) and with a variable rate of fall (approximately within 45-70 m/s). The first ones include group acrobatics and beginner parachutist training (acquisition of stable fall skills). The second ones include artistic forms of the parachuting, such as: ‘free fly’, ‘free style’ and others.
For an effective training of a parachutist or of a group of parachutists in the working of the wind tunnel, it is necessary that the air-resistance force applied to the parachutist's body be equal to the force of his/her weight. Otherwise, the inequality of the mentioned forces will cause a longitudinal displacement of the parachutist along the axis of the flow and his/her exiting the working zone or encountering the safety net due to the limited working.
The most reasonable solution to keep the parachutist or group of parachutists within the working is to design the working as a diffuser with an entrance velocity corresponding approximately to the upper limit of the fall velocity range and an exit velocity corresponding approximately to the lower limit of the fall velocity range.
At the same time, due to the negative gradient of the velocity flow along the axis of the working, self-stabilization the body's position occurs, i.e. the body is positioned at a level within the working zone, where the influencing air-resistance force equals to its weight. The change of the master cross-section or of the air-resistance coefficient (in case of the position change) causes the body's transfer to a new equilibrium level.
It is known training equipment for parachutists with a diffuser-type working, possessing a velocity rate within 45-70 m/s, (see Reference-1). The working of the given training equipment comprises two sections, between which sections a net can be placed.
During the performing of artistic parachuting exercises, the sections separating net is not placed, and the parachutists can move from the lower to the upper limits of the working.
When performing the group acrobatic exercises or during the beginner training of the parachutists, the net is placed and the parachutists exercise in the upper half of the working, in the section, where the velocity of flow is approximately 50 m/s.
The main disadvantage of this equipment is unsatisfactory quality of the flow in the second section that negatively influences the quality of the group acrobatics teams' training and causes the habituation to harmful reflexes, which affect negatively the teams' results in performing the parachute jumps. It is based on three reasons:
Firstly, for speeding-up the performing of some group acrobatics figures, the spinning of parachutists is executed in such a way that one parachutist passes under the body of another parachutist, so during their rearranging the parachutists are on different levels, but at the moment of transition they move with the same vertical velocities.
When performing such a transition in the working of the wind tunnel with a longitudinal gradient of velocity, the parachutist who flies up higher appears within the section where the flow speed and, correspondingly, the dynamic pressure are less; whereas the parachutist who descends lower appears in the section where the flow speed and, correspondingly, the dynamic pressure are greater. To compensate the flow velocity change, the upper parachutist has to increase the master cross-section of his/her body and the lower parachutist has to diminish it, correspondingly. The intensive trainings in the wind tunnel lead to the situation when the mentioned actions to compensate the dynamic pressure change become reflex.
When performing the parachute jump in the free air, the longitudinal gradient is absent and the established reflex habit for its compensation by the increase or decrease of the body's master cross-section causes that during the spinning performance with crossing planes the upper parachutist ascends higher than it is needed and the lower parachutist descends lower than the required level, that leads to vertical dispersion of the parachutists and, consequently, worsens the results.
Secondly, the presence of the initial diffuser section causes the velocity profile change, the boundary layers thicken and the steady speed core diminishes. The velocity profile becomes convex-shaped. At the same time a greater dynamic pressure influences the part of the body that is closer to the flow's axis and a lesser dynamic pressure influences the part of the body which is further from the flow's axis. It produces the twisting moment, which changes the body's angle of incidence that causes a deviation of the resultant aerodynamic force towards the flow's periphery, and, in consequence of this, the lateral component of the resultant aerodynamic force pushes out the body to the flow's periphery. This lateral force equals zero on the axis of the flow and increases while moving away from the axis.
Thirdly, resulting from the diffuser having an opening angle, flow velocity vectors' directions within the working section are not parallel. In the centre of the working the flow velocity vector is parallel to the gravity force vector, whereas next to the working wall, there is an angle between the flow velocity vector and gravity force vector, which angle equals a half of the diffuser opening angle. Correspondingly, the resultant aerodynamic force for a symmetric body is directed strictly upwards in the centre of the working and angularly in the periphery zone. The more the body comes closer to the working wall the greater is that angle. That's why, the lateral force acts upon a body situated out of the axis of the flow, which lateral force pushes out the body to the periphery of the flow. This lateral force equals zero on the axis of the flow and increases while moving away from the axis.
As a result of adding the lateral forces mentioned above, an effect of working on a spherical surface appears, when the body is constantly going to “slide” towards the working wall.
To compensate the mentioned effect, the parachutists make their body to incline in relation to the working direction. The intensive trainings in the wind tunnel lead to the situation when the mentioned actions to compensate the lateral force become reflex. When performing the parachute jump in the free air the lateral force is absent, and the established reflex habit for its compensation by inclining the body to the centre of the group result in that the team works too densely, “bunches up”, that negatively affects the results of its performance.
One more disadvantage of the mentioned training equipment is excessive hydraulic losses when working in the upper section of the working, because the lower diffuser section unused at this time, the lower safety net, and the honeycomb, placed in the narrowest section of the channel, are the main sources of the hydraulic losses.
The mentioned disadvantages can be eliminated by designing parachutists training equipment, which comprises two working zones, each one adapted for the most effective training for every parachuting type.
The working zone, designed for group acrobatics and beginner training exercise of parachutists, has to possess a minimum length of the initial section and a minimum negative velocity gradient. The length of such working zone is, correspondingly, relatively short (about one caliber), and the opening angle is about a few degrees (0.5-5°).
A small longitudinal velocity gradient in the working zone reduces the required compensation of the dynamic pressure change on the different levels when performing the spinning with planes crossing. A relatively small length of the working zone excludes increasing the boundary layer to a significant thickness and helps maintaining a steady speed core, whose dimensions are close to the diameter of the working zone that eliminates a first component of the lateral force. A relatively small opening angle of the working zone reduces the deviation of the resultant aerodynamic force on the periphery of the flow that decreases a second component of the lateral force.
The working zone, designed for artistic parachuting exercising, has to possess corresponding velocity rates at the entrance and at the escape zones, and the length of the working zone must amount to about 2-4 calibers at an opening angle ranging from 5 to 10°. The presence of the velocity gradient and the lateral force gradient is less critical in this case, while the velocity rate is decisive, and thusly the length of the working zone and the opening angle are chosen taking into consideration the minimization of hydraulic losses in the wind tunnel contour and technological limitations.
The mentioned working zones may be placed in the training equipment contour in parallel, sequentially, or they may be removable. The most effective is the sequential design of the working zones.
Known is parachutists training equipment with several working zones, placed sequentially on one axis (see Reference 2).
The main disadvantage of the mentioned equipment is the placement of ventilators (i.e. fans creating an air pressure difference) in the sections of the channels between working zones and the absence of elements, which can equalize the velocity distribution at the entrance of the working zones. A necessary element of the ventilator is a bushing that in the modern ventilators has a diameter from 0.4 to 0.7 of the channel's diameter, wherein the ventilator is installed. This bushing creates a considerably wide and long wake (trace).
The presence of the ventilator bushing's wake in the working zone makes the valid training impossible, because getting of the parachutist into the wake, where the dynamic pressure is lower than in the main flow, will cause his/her fall on the safety net. On the other hand, movement of the parachutist within the working zone without getting into the wake is practically impossible, because it occupies a significant part of the working zone's diameter. Moreover, the ventilators' bushings, placed in the sections close to the working zones' sections by their areas, with taking into account the flow's squeezing on the bushing, will become sources of significant hydraulic losses.
The mentioned disadvantages can be eliminated by creating a wind tunnel, whose channel doesn't contain elements, worsening the steadiness of the velocity distribution that causes local flow turbulences and creates significant hydraulic losses.
The most technically close to the proposed technical solution is a wind tunnel, designed for aerodynamic research (see Reference-3, page 177), comprising a channel created by sequentially placed and interconnected: an entrance contraction (confusor), a first working zone, an intermediate contraction, and a second working zone.
The main disadvantage of the mentioned wind tunnel is the impossibility to train simultaneously in both working zones (since elements, excluding the getting of the models wake from the first working zone into the second working zone, are absent), resulting in the researches are made in the working zones separately, taking turns.
Therefore, while the research is conducted in one working zone, the flow also passes through the other working zone, not used at the moment, and creates additional unwanted losses, especially in case of working within the first working zone, because the additional squeezing in the intermediate contraction, with the following braking in the contraction after the second working zone increases significantly common hydraulic losses in the contour.
A disadvantage of the mentioned wind tunnel, related to the sportsmen parachutists training, are the absence of a velocity gradient in the working zones (the velocity in the steady speed core is the same in any working zone's crossing).
Another disadvantage is the impossibility of simultaneous work of the parachutists in the first and in the second working zones, resulting from the significantly different velocities. So, if an equilibrium velocity rate is set in the first working zone, in the second working zone this rate will be greater than the equilibrium, hence the parachutists will be pushed out by the excessive dynamic pressure to the diffuser, or will be pressed against an upper safety net (if its installation is arranged). If the equilibrium rate is set in the second working zone, the rate in the first working zone will be less than the equilibrium, thus the parachutists will fall into the entrance diffuser, or will fall on the lower safety net (if its installation is arranged).